The dogma that fully differentiated somatic cells have absolutely irreversible properties was generally accepted for a long time. This began to change when a series of pioneering experiments showed that silent gene expression profiles can be completely reactivated by the fusion of different pairs of cell types (Blau, H. M. How fixed is the differentiated state? Lessons from heterokaryons. Trends Genet. 5, 268-272 (1989)). More recently it was shown that transfer of nuclei from a somatic cell type into an enucleated egg cell could lead to the complete reversion of the somatic cells' gene expression profile, and to the formation of a pluripotent cell state able to generate new entire animals (see e.g. Gurdon, J. B. & Melton, D. A. Nuclear reprogramming in cells. Science 322, 1811-1815 (2008)). Yamanaka and colleagues (Takahashi, K. & Yamanaka, S. Induction of pluripotent stem cells from mouse embryonic and adult fibroblast cultures by defined factors. Cell 126, 663-676 (2006)) demonstrated that somatic cells can be reprogrammed to induced pluripotent stem cells (iPSCs) by transduction of four defined factors (Sox2, Oct4, Klf4, c-Myc). Different types of somatic cells including fibroblasts, keratynocytes and adipocytes have been reprogrammed to an iPSC pluripotent state. During the past years the question arose whether specific somatic cell types could be transdifferentiated to a completely different somatic cell type such as a neuron. Wernig and colleagues addressed this question showing the direct conversion of mouse fibroblasts to functional neurons by transduction of three crucial genes: Mash1, Brn2 and Myt11 (Wernig at al. Direct conversion of fibroblasts to functional neurons by defined factors. Nature 25; 463(7284):1035-41 (2010). However the neurons obtained are postmitotic cells which are by definition not able to proliferate and which do not tolerate freezing and thawing procedures. US2010/0021437 discloses a method for generating induced pluripotent stem cells from fibroblasts and inducing those cells to differentiate into neural phenotypes.
However, direct conversion of differentiated somatic cells to neural stem cells has not been described so far. Neural stem cells are multipotent stem cells and are reported to be propagated under specific conditions. They require a chemically defined medium, for example N2B27 medium (N2B27 is a 1:1 mixture of DMEM/F12 (Gibco, Paisley, UK) supplemented with N2 and B27 (both from Gibco)) supplemented with FGF (fibroblast growth factor 2) and EGF (epidermal growth factor). They can grow as a monolayer adherent culture, e.g. on Poly-ornithine/Lamin coated plate or as floating neurospheres in non-adherent cell culture plates. The two types of neural stem cell cultures (neurospheres, adherent cultures) have been reported to be completely inter-convertible. Neural stem cells can be grown indefinitely and still remain truly multipotent. Upon special conditions they differentiate into the cell types that compose the adult brain, including neurons, astrocytes and oligodendrocytes. Neural stem cells are considered possible therapeutic agents for treating patients with neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease, stroke, and spinal cord injury.